Unloader.



L. A. GRAY.

UNLOADER.

APPLICATION min MAR. s. 1916.

Ill/[ 7157 5 SHEETS-SHEET 1 Patented N0v."28, 1916.

WIT/V588 J35. @VA

' ATTORNEY L. A. GRAY,

UNLOADER.

APPLICATION FILED MAR. 3, 1916 I 1 ,206 53%, Patented. Nov. 28, 1916.

5 SHEETS-SHEET2- I ll 41 q J2: +1 i 1% y 45 1|? u i FEW I, M 1 l 1 96 95 f 7 WITNESS INVENTOR L. A. GRAY.

UNLOADER.

APPLICATION flLEDMAR. 8. I916.

Patented Nov. 28,1916.

5 SHEETS-SHEET 3.

' )1 TTOA/EY L. A. GRAY.

UNLOADER.

APPLICATION FILED MAR-8,1916.

Patented Nov. 28, 1916.

5 SHEETS-SHEET 4.

ATTORNEY L. A. GRAY.

UNLOADER.

APPLICATION man MAR. 8, I916.

1,206,532. Patented NOV. 28,1916.

5'SHEETSSHEET 5.

I H i 39 I r n F 12 k F 15 I fl a 1 L 92 95 13 92 5:; a u; 5 i1 94 1 i WITNESS Ill/I/E/VTOR BY Z ATTORNEY passed through the we rem ur e t a all or ii.

LAWRIEITCE it. GRAY, OF MULLAN, IDAHO.

UNLUADER.

Specification of letters Patent.

Patented Nom'fZS, isle.

Application filed March 3, 1916. Serial No. 32.788.

multi-stage air or gas compressor, which au-- toinatically relieves the compressor of load when the pressure in the receiveror delivery line reaches a predetermined value and causes the compressor to take up full load again when the pressure falls below a critical point, and which obtains these results without loss of volumetric eliiciency or power and without loss of compressed air trapped in the intercooler.

As distinguished from other unloaders of similar location, my invention is dependent neither upon a throttling oi the intake, which causes overheating and loss of volumetric eiticiency, nor upon the production of a vacuum in the compressor, which presents serious mechanical difliculties and also results in heating and loss of power it a practical vacuum be not obtained. My unloader closes the intake of the compressor without throttling when the receiver pressure arr'ves at the predetermined value, and thereupon places the intake in communication with the intercooler, so that thereafter the piston of the low pressure stage operates idly, taking air from and returning it to the intercooler, while the high pressure stage by its continued operation draws upon the partially compressed. air stored in the in tercooler until the pressure in the latter falls substantially to atmospheric conditions: then the intake of the low pressure stage is reopened to the atmosphere, so that, during the succeeding period and until the receiver pressure falls below the critical value, atmospheric air is low pressure stage with out having work done upon it, thereby lieving the compressor of a large part of its load and cooling the low pressure stage by the circulation of air therethrough. The high pressure stage now operates as a single' stage compressor, receiving at atmospheric pressure from the intercooler; it is to be understood, however, that additional means may be provided, if desired, for also unloading the high. pressure stage. "When the pressure in the receiver or delivery line becomes less than the critical value, the communication between the intake of the low pressure stage and the intercooler inserted between the stages is closed, and, the intake remaining'open to the atmosphere, normal load conditions are restored. A preferred embodiment of the unloader, comprising a combination of main valves, motors for operating the same and pilot valves and con nections for controlling the operation of the motorsso as to produce full opening and closing of the main valves in a predetermined order, dependent upon the conditions of receiver and intercooler ressures, will now be briefly described, but/t is to be understood that numerous modifications are permissible and that the essential features of the invention may be embodied in other specifically diii'erent forms and combination.

In the accompanying drawings: Figure 1 is a vertical section through the unloaderv casing and valves, and through the casing containing the valve-operating pistons and pilot valves, the latter being shown partly in section and partly in elevation. This view shows the parts in the positions which they hold during the normal operation of the compressor. Fig. 2 is a vertical section on the line 2-2 of Fig. 1. Fig. 3 is a semi-diagrammatic plan view, showing how the unloader may be arranged with reference to the other parts of the compressor system. Fig. 4 is a horizontal section through the unloader casing and valves. Fig. 5 is an onlarged plan View of the upper casing and piping connections. Fig. 6 is a side elevation of the same. Fig. 7 is an enlarged elevation of pilot valve 47. Fig. 8 is an enlarged vertical section through pilot valve 48 in its chamber. Fig. 9 is an enlarged vertical section illustrative of another arrangement of springs for the main valves of unloaders of large size. Fig. 10 is a vertical section through the upper casing, the pistons and pilot valves within all being down. This is the condition which results when the receiver pressure has risen above the predetermined value, thereby forcing down the pilot valve l8, with the result that pistons- 39 and 4-0 are'forced downward in succes sion. 11 is a similar view, with piston 39 and pilot valve 47 up and pilot valve 48 and piston 4:0 still down. This is a condition succeeding that of Fig. 10, the inter suitable niotors, con'zprising vand 40 are loosely moan cooler pressure having fallen sufficiently to pern n't valve 47 to rise, this valve then pernetting air to exhaust from cylinder 41 so that piston 39 could also rise. Figs. 12 andpressure stage and 16 the high pressure stage. 'lhe unloader, designated as a whole hy the numeral 1'2, is connected in the inlet pipe 3.8, as near as possible to the intake 19 or the low pressure stage. 520 is the outlet 2 from low pressure stage to the intercooler 21 and is the pipe leading i n the intercooler to the intake of the high pressure stage, iron which the finally cont pressed air is delivered through pipe 23 to a receiver 24 connected with the high pressure line 25. c

As illustratedpthe' unloader comprises a 26 having three connections, 2?, 28 29, 2? being the inlet connection, 28 the ntlet delivery to the intake of the low pressure stage, and 29 being connected with the inter-cooler through pipe 30. lnteriorly the casing is divided by suitably arranged Walls 31 to form three chambers open to the three connections, the chambers thus formed being an inlet or source chamber 32, an outlet chamber 33 communicating with compressor intake, and a chamber 34 communicating With the iIr'c-ercooler For onvenience, therefore, vthese three charm Mrs-may he termed, respectively, the source amber, the intake chamber, and the interoooler chamber. The top and hottom Walls of the intake charnher are formed i with; openings for two main double-seat valves 35, 36, the cooperating seats being designated 37' and 538. During normal operation the valve 35 remains open an the valve 36 closed, These valves are operated by pistons and 40 operating'in fluid pressure cylinders 41, 42 against the action 11npressicn springs 43, 44 surrournling,- the val rods 45 and 46, on the upper ends of which the pistons 39 l Under normal conditions these springs .o. the pistons an d the valves 35 and Sill: their highest positions, in which positions the valve 35 is open and the valve is closed. When fluid pressure is admitted to the cylinders 41 and 42, above the pistons, the latter are driven downward, thereby seating valve 35 and opening valve 36, provisions being made, however, as presently described, whereby the second valve is not opened until after the first has been closed.

The operation of the fluid pressure pistons 39 and 40 is controlled by two pilot valves 47 and 4-8, i-

nature of pistons, operating in cylindr l cnamhers 4a and 5Ov vent 53 the tuhular extension 'sitioned to register With the The top of the chamber 50 is in coromunication With the receiver 24 through a small pipe 51, and the chamber 49 is similarly connected with the intercooler by a pipe 52, cylinders and chambers 41, 42, 48 and 49 are preferably formed in axial parallelisin' in a block or casting 53, supported from the top plate 54 of the unloader casing by of stanchion rods 55 and having its corn by plate 56, he pilot v 1 I a 57, l 'e and eng; 58 which ;..ive upward loo 48 shown mounted on shown in l "'3', as irei'erably constructed, '5; cola coin; ression spring 59 housed in toe interior of the said valve and in. a lined tubular extension 60. iorceof this spri be adjusted by ascrew 61, and in like manner the tension. of the springs 43, 4' and may be regulated by setti g collars at ditlerent points on the loo he rods 45,

' 57 pass loosely thrc A at the downward move: one 39 and and of the v opposed by the a oer sphere through a The pilot valve chamber in addi on to the inlet inits ton fr in receiver, is provided in side with sport 64 COi'llllllsllh eating with a pipe 65, this port being in such position that it is uncovered by the valve 48 When tl' ,ter is its uppermost position only,

l t 1e pilot valve char; is provided in op asste sides and at the same level With ports 66 and 53, which. when the pilot lve 47 is in its lowermost position (seen v l and 10) register with an circumferential valve channel 68. This valve is also provided with lateral exhaust port 69 formed throu 'n. .ELS wall and positioned to register with the ports 6? when the valve is in its uppermost position; and, in order to insure the registrv he pilot e ng may.

n closed top, and is kept open. by p of these ports at such time, means are pro-' vided, as indicated at 70, to lreep the valve from turning. The valve 48 is also hollow (see Fig. 8), being closed at the bottom and open atthe top, and is provided with a -lateral port "Z1 leading from its n erior to is powhen we top ver .a, and when a circinnl erential channel the valve is in its lovvcstpo this valve, a, pressure acts,

reduced a the valve is in its upper, no mal position,

the air pressure is lrcpt from acting on r of the v" ve beca se of the prol s 81) on the L s r 41 has ia-I and 7d at opposite sides, the one located at its top and the other in a lower position to be uncovered by the piston 39 when the latter is forced downward. The cylinder l2 is also provided with two ports 7 and 76, the port 75 being at the top and the port 76 111 position to be uncovered b 1 the piston 40 directly after the latter starts on its downward movement. The pipe 65 leading from port 64 of pilotvalve chamber 50 connected .on one hand with port 66 of valve "chamber 49 through a connection 77, and i with port 73 of interchangeable with respect thereto,

bringing the intercooler on the other hand through piping 7 8 with port 76 of fluid pressure cylinder 42-. Port 67 of pilot valve chamber l9 is connected fluid pressure cylinder ell by a pipe 79. From port 74: of this cylinder fluid may pass through piping 80 having an.

automatic non-return valve 81 to port 75 at the top of fluid pressure cylinder 72.

.. The lower portions of the main valve rods and 46 pass through a bottom plate 82 of the unloader casing and carry pistons 83 and 84 which operate in dash-pot cylinders erably of duplicateconstruction at top and bottom, and its closure plates 54 and 82 are so that be inverted, thus connection 29 at either'the right or the left-hand side, as may the body of the casing can .be made desirable by the layout of the To invert the body plant; .and for the same purpose the valve seats 37 are removable and reversible with re spect to the valve seat openings in the walls 31. To this end the seats 37 and 38 are formed in removable. bushings 89 and 90, which comprise ring portions 91 having the seats ground therein and narrow longitudinal portions 92 connecting the rings.

of the casing the unloader is first taken apart and then the body is turned upside down about a. horizontal axis perpendicular to the plane of Fig. 1. This brings the intercooler connection at the left instead of at the right and the positions of valves 35 and 36 are interchanged. Now, in order that the valve seats may face the valves, it is necessary to invert them in their openings. When the closure plates are replaced, the plate 54: may still occupy the position at the top and the plate be at the bottom because of the duplicate construction of the ends of the body. It is necessary, however, to reverse the closure plates, so that their righthand ends are at the left and vice-versa, in order to bring the valve rod openings in vertical alineinent with the new positions of the valves.

also necessary to reverse the block 53 containing the pilot valves.

ilhe valves 35 and 86 are shown as being built up in the same manner and the description of the preferred construction of one will suilice for both. The valve 85 comprises two end rings 98 and 9?: ground to coiiperate with the seats 3'? and united with each other by longitudinal portions into these ring portions are screwed heads 96 and 9? having central apertures closely f tting the rod 15 and when in position abutting against shoulders formed by an enlargement 98 on the rod. Additional vensile strength is afforded the valve by steel screw bolts 99 which engage threaded openings in the heads 96 and 97 The operation or" the particular embodiment of the invention illustrated will now be briefly described.

Duringthe normal operation of the com presscr, the condition of the parts is that shown in Pg. 1; that is to say, the main valve 35 is open, the other main valve 36 is closed, their pistons and t0 are both in their upper positions, the pilot valve 48 is in upper position, and the pilot valve l? is down. It now the receiver pressure rises above the predetermined value, this pressure acting upon the head of the pilot valve l8 depresses the latter against the action of spring; 58 until the valve bottoms against the plate 56, when the groove registers 1 the port (ll. This air is then free to pas through connections and '77 to ports 66 pilot valve chamber 49. This valve being down, it may flow around through the circumferential channel 88 to the port 67, whence it passes through the pipe '?9 to port T3 at the top ot' cylinder 4L1. lhe pressure acting upon. the top of piston 39 in this cylinder forces the latter downward against the tension of spring 4-8 thereby closing the valve upon its seat. When this occurs the piston 39 has uncovered the'pcr 74, so that the motive fluid may flow by way of pipes 89 to port at the top of fluid pressure cylinder 4-2. In this the piston to is forced downward to open the main valve 38, but not until after the main valve is closed. V' hile this condition continues, the intake ot' the low pressure stage of the compressor is cut oil from the atmosphere, so that no more atmospheric air is taken into the compressor, but instead the intake is placed in cominuniction with the intercocler to which the low pressure stage delivers, so that the piston of this stage operates idly, drawing air from and returning; it the intercooler without doing work thereon. The cylinder of the high pressure stage the con'ipressoris meanwhile taking" air cm the intercooler, so that the pressure in latter falls, and when it is reduced to desired degree or practically to atmospheric pressure, the pilot valve 47 rises, because the intercocler pressure acting through pipe is no longer strong enough to overbalance the spring 59. This upward movement of the pilot valve at? interrupts the fluid pressure connection between pilot valve chamber 50 and the fluid pressure cylinder 41, and instead brings the exhaust port 69 into registry with port 6?, with the resultthat the space above the piston 39 is placed in communication with the atmosphere, so that the spring as promptly reopens the main valve 35. The fluid pressure cylinder 42, however, is not exhausted, the valve 81 closingagainst any back flow through the pipe 80; and fluid pressure continues to act upon the piston 40 through the port 76 which was uncovered when the said piston moved downward. Therefore, the piston 40 is not allowed. to rise and the main valve 3% remains open. Now both the intake and the intercooler are open to the atmosphere, so that the piston of the low pressure stage of the compressor continues to operate idly, circulating atmospheric air through its cylinder. The

high pressure stage from now on operates as a. single stage compressor, receiving at inospheric air from the intercooler, compressing it and delivering it into the receiver. However, as stated, my invention does not necessarily exclude the provision of suitable means for unloading the high pressure stage well as the low pressure stage. The circulation. of atmospheric air through the low pressure stage during this period together with the action of the usual water-jacket has a decided cooling effect upon this stage, so that for a time after the normal operation is resruued the volumetric efliciency of the compressor is substantially increased. It may now be assumed that the receiver pressure falls below the critical value, thumpermitting the pilot valve 48 to rise. This cuts oil cylinder 42 from the supply of motive fluid and permits the same to exhaust to the atmosphere through connections 7S and 65 when the lower end of the valve uncovers port G l. This enables spring ll to raise piston so and close the main valve ln this way the normal condition is restored and the compressor re sumes full load, its in he being open to the atmosphere but out o from the l l lbci cooler.

C ne other condii 1 remains to be con sidered. T the contingency that the receiver prr ire may fall below the critical value before the action of the piston of the high pressure stage of the compressor has reduced the pressure in the intercooler to that of the atmosphere, or to such low point as will permitthe pilot valve 47 to be raiser.

by its spring 59 or the action of atmospheric pressure below said valve. The result of the upward movement of pilot valve d8 unloaders of large size,

under the condition indicated'is to out off both of the cylindersll and 42 from fluid pressure and to connectboth of them with the atmosphere, so that thei'r 'pistons .may rise under the action of 'the spi'ings 43' and 44. The exhaust passage fromizylinder 42, however, is much more direct and may also be of larger cross-section than that from cylinder ell, and the spring 44 is preferably stronger than the spring 43, so that the main valve 86 is seated before main valve is reopened, thus avoiding loss of such pressure as remains in the intercooler. In this connection it may be noted that the ends of the main valve 36 are of unequal area, the arrangement being such that the pressure left in the intercooler has an increasing effectto close this valve the nearer it approaches its seats. Consequently the closing of the port 76 by the piston when the latter has almost reached its upper position does not check the closing of the main valve 36. The clearance above the piston 40 is so greatly augmented by the interior of the piping 80 that the compression of air by the said piston after closing port 76 is negligible.

The purpose of reducing the area of the pilot valve 48 initially exposed to receiver pressure and of the provision of the seat 80, is to cause this valve to yield only to an excess pressure in the receiver, that is to say a pressure greater than that necessary to continue the downward movement of the valve and to hold it down, after its movement has once been started. This obviates fluttering of the valve. It also results that the valve returns to normal position at a somewhat lower receiver pressure than that which is required to move it from normal position. 1

Fig. 9 shows a slight modification in respect to the springs which restore the main valves to their normal. positions. This arrangement is suitable more particularly for for which extension springs are more desirable than. compression springs. iwo such springs 43 are shown at opposite sides of the valve rod being adjustably connected at their upper ends by means of eye bolts 100 to a cross-piece 101 on top of the block 53, their lower ends being secured to a cross-piece 102 bearing against an adjustable collar or nut 103 on the valve rod.

The compressorgihes been referred to for convenience as an air compressor, but it will be understood that the invention is equally applicable to gas compressors. The term atmosphere therefore means merely the source from which the compressor draws the fluid to be compressed, and atmospheric pressure means the pressure of the source. The term receiver pressure is employed as a convenient one meaning pressure at [11+ delivery side of the compressor, or a point beyond the stage which operates upon the partly compressed fluid from the intercooler. This term is not intended, therefore, to limit the invention to the employment of a receiver.

What I claim as new is:

1. The combination of a multi-stage air 1 gas compressor having an intercooler, and means controlled by receiver pressure and operative when the same reaches a predetermined value to shut .03 the intake of the low pressure cylinder from the source and thereupon to place said intake in communication with the intercooler.

2. The combination of a multi-stage air or gas compressor having an intercooler, means controlled by receiver pressure and operative when the same reaches a predetermined value to shut 0a the intake of the cation with the intercooler,

low pressure cylinder from the source and thereupon to place said intake in communication with the intercooler, and means controlled by the intercooler pressure and operative to connect both the intake and the intercooler with the source when the pressure in the intercooler has been reduced approximately to that of the source by the continued operation of the high pressure cylinder piston.

3. The combination of a multi-stage air or gas compressor having an intercooler, means controlled by receiver pressure and operative when the same reachesa predetermined value to shut oi the intake of the low pressure cylinder from the source and thereupon to place said intake in communication with the intercooler, and means connected with the intercooler and operative upon a predetermined lowering of pressure therein to re-open the intake to the source, the communication between the intake and intercooler remaining open.

4. The combination of a multi-stage air or gas compressor having an intercooler,

means controlled by receiver pressure and operative when the same reaches a predetermined value to shut off the intake of the low pressure cylinder from the source and thereupon to place said intake in communication with the mtercooler, means connected with the intercooler and operative upon a predetermined lowering of pressure therein to reopen the intake to the source, the communication between the intake and inter-' cooler remaining open, and means whereby the said communication is closed when the receiver pressure falls below a critical value.

SQThe combination of a multi-stage. air or gas compressor having an intercooler, means controlled by receiver pressure and operative when the same reaches a predetermined value to shut off the intake of the low pressure cylinder from the source and thereupon to place said intake in communimeans connected with the intercooler and operative upon a predetermined lowering of pressure therein to re-open the intake to the source, the communication between the intake and intercooler remaining open, and means whereby the communication between the intake and the intercooler is closed and that between the intake and source re-opened, in event I of the receiver pressure falling to a critical value before the-supply of compressed fluid trapped in the intercooler has been ex hausted.

6. An unloader connected with the intake of a stage of a multi-stage air or gas compressor and comprising two valves, the first of said valves constructed to open and close communication between said intake and the source and the second to open and close communication between the intake and the intercooler to which said stage delivers, and means controlled by receiver pressure and operative when the same reaches a predeten mined value to close the first valve and then to'open the second valve.

7 An unloader connected with the intake of a stage of a multi-stage air or gas compressor and comprising two valves, the first of said valves constructed and arranged to open and close communication between said intake and the source, the second to open and close communication between the intake and the intercooler to which said-stage delivers, means controlled by receiver pressure and operative when the same rises to a predetermined value to close the first valve, and means dependent upon the closing of the first valve whereby the second valve is opened.

8. An unloader connected with the intake of a stage of a multi-stage air or gas compressor and comprising two valves, the first of said valves constructed and arranged to open and close communication between said intake and the source, the second to open and close communication between the intake and the intercooler to which said stage delivers, pistons connected with said valves,

and fluid pressure connections whereby said the intercooler to which said stage delivers,

pistons connected with said valves, fluid pressure connections whereby said P18130118.

are operated when the receiver pressure rises cooler, valve is again opened, the second valve reth y 1 ting the intake with the said ,5. Jntercooler,

and further connections whereby the first valve is again opened, the second valve remaining open, when the compressed fluid in the intercooler has been sufliciently exhausted.

10. An u'nloader take of a stage of:

connected with the ina multistage an or gas compressor and comprising two valves, the

first of said valves constructed and arranged to open and close communication between the. intake and the source, the second to open and closecommunication between the intake and the intercooler to which said stage delivers, pistons connected with said valves, fluid pressure connections whereby said pistons are operated when the receiver pressure rises t a predetermined value-to close the first valve, thereby shutting off the intake from the source, and to open the second valve, thereby connecting the intake with the intercooler, whereby at this time the next higher stage of the compressor exhausts the compressed fluid trapped in the intercooler and the lower stage operates idly, taking fluid from and returning it to the interfurther connections whereby the first mai'ning'open, when the compressed fluid in the intercooler has been sufliciently exhausted, so that thereaftergthe higher stage of the compressor receives fluid at the pressure of the source while the loWer stage is cooled by the circulation of fluid therethrough, the first mentioned connections causing the second valve again to close, the first valve remaining open, when the receiver pressure falls below a critical value, thereby restoring the normal condition.

11. -An unloader connected with the intake of a multi-stage air or gas compressor and comprising two valves, the first of said valves constructed and arranged to open and close communication between the intake and the source, the second to open and close communication between the intercooler to which said stage deli'vers, fluid pressure means for operating said valves, and means for controlling the operation of the same, including connections with a source of pressure at the delivery side of the compressor whereby the first valve is closed and the second thereupon opened when such pressure rises to a predetermined value, connections with the intercooler of the compressor wherebythe first valve is opened, the second remaining open, when compressed fluid trapped in the intercooler has been sufficiently exhausted, 0nd valve is again closed, the first remaining open, when the receiver pressure falls below acritical value, and means whereby the sec.-

intake and the means whereby the sec- 0nd valve is closed, and the first thereupon opened in event of the pressure at the delivery side falling to the critical value before the compressed fluid trapped in the intercooler has been exhausted.

12. An unloader connected with the intake of a stage of a multi-stage compressor and comprising two main valves, the first adapted to open and close communication between said intake and the source, and the second to open and close communication between the intake and the intercooler to which said stage delivers, fluid pressure cylinders and pistons for the operation of said valves, and two pilot valves controlling the operation of said pistons, one of said pilot valves being operated by receiver pressure and the other by intercooler pressure.

13. An unloader connected with the intake a stage of a multi-stage compressor and comprising two main valves, the first adapted to open and close communication between said intake and the source, and the second to open and close communication between the intake and the intercooler to which said valve.

14. An unloader connected with the in take of a stage of a multistage compressor and comprising two main valves, the first adapted to open and close communication between said intake and the source, and the second to open and close communication between the intake' and the inter-cooler to which said stage delivers, fluid pressure cylinders and pistons for the operation of said valves, a pilot valve and a line whereby the same is controlled by receiver pressure to admit fluid to the fluid pressure cylinder of the first main valve, a connectionbetween the said cylinder and the fluid pressure cylinder of the second main valve, and a second pilot valve and a line whereby the same is controlled by intercooler pressure, the said second pilot valve being interposed in the passage between the first pilot valve and the fluid pressure cylinder of. the first main valve and having means for connecting said cylinder with an exhaust, there being a check-valve in the connection between the connection with the intereooler to An unloader connected with the in take of a stage of a multistage compressor and comprising two main valves, the first 4 rated to open and close communication ween said intake and the source, and the and to open and close communication been the intake and the intercooler to which s id stage delivers, fluid PlQSSUlG cylinders pistons for the operation of said valves, sa 1 iluid pressure cylinders being connected in seriesby a passage having a cbeclcvalve therein, a pilot valve controlled by receiver pr ssure tor controlling the admission of tluid pressure to the fluid pressure cylinder of the first main valve, a second pilot valve controlled by intercooler pressure, the said second pilot valve being interposed in the connection between the first pilot valve and the fluid pressure cylinder of the first main va ve; and being adapted upon decrease in *coolcr pressure to interrupt said con-- tion and to exhaust the said fluid pressu cylinder, and a further connection be tween the first pilot valve and the fluid pressure cylinder of the second main valve whereby to hold the piston in this cylinder in actuated position alter the return of the other piston and to exhaust the said cylinder when the receiver pressure falls below the critical value.

l6. unload-er connected with the in receiver pressure and a transverse port communicating with its interior, a passage with which said transverse port communicates when the said valve is actuated, said passage leading to the fluid pressure cylinder of the first main valve, a second pilot valve in a valve chamber connected with an intercooler line, said second pilot valve having a circumferential channel forming part of the lastinentioned passage and having also a hollow interior and a transverse exhaust port adapted to exhaust the fluid pressure cylindcr of the first main valve when the intercooler pressure has been reduced to a predetermined value.

1'7. An unloadcr designed for connection with the intake of a stage of a multistage air or gas compressor, comprising a casing having walls therein forming a chamber open to the source, a chamber open to the said intake and a chamber having means ll iltfi i'l said stage delivers, a valve constructed and arranged to open and close communication between the source and intake chambers, a second valve constructed and arranged to open and close communication between the intake and intercooler chambers, and means operating said valves, substantially as described.

An unloader designed for connection with the intake of a stage of a multi-stage vspect to said cylinders.

19. The combination with a multistage air or gas compressor, of an unloader connected with the intake thereof and comprising main valves adapted to open and close communication between the intake and the source and between the intake and the intercooler, respectively, motors for operating said valves, and pilot valves exposed to receiver and intercooler pressures, respectively, and controlling the operation of said motors, substantially as described.

20. An unloader connected with the intake of a stage of a multistage compressor and comprising two main valves adapted to open and close communication between said intake and the source and between the intake and the inter-cooler to which said stage delivers, respectively, fluid pressure cylinders, pistons and passages for operating said valves, a pilot valve responsive to receiver pressure controlling passages of both cylinders, and another pilot valve controlling passages of one of the cylinders and responsive to intercooler pressure.

'21. An unloader for connection with the intake of a multi-stage air compressor, the same comprising a casing formed at opposite sides with an inlet from the source and an outlet to the intake, and being further provided with. an intercooler connection at right angles to said inlet and outlet, walls in said casing dividing the same into three chambers open, respectively, to said inlet, outlet and intercooler connection, and two double-seat valves in said casing cooperating with seats in said walls to control communication between the chambers.

22. An unloader for connection with the intake of a multi-stage air compressor, the same comprising a casing having an inlet connection, an outlet connection to the intake of the compressor, and an intercooler connection, and being divided internally into three chambers open to the three connections, and tWo valves in said casing controlling communication between the three chambers.

23. An unloader for multi-stage compressors, comprising a body having interchangeable top and bottom heads, three external connections and Walls dividing its interior into three corresponding chambers communicating with each other by openings 10 for double seat valves valve seat bushings LAv'vRENoE A. GRAY. 

